Heretofore, acoustic emission detection techniques have been utilized for investigating mechanically-generated sounds such as produced by deformation and fracture, fatigue cracking and residual stress, and mechanically loading welds and the like. This is done by placing a sensitive microphone on the structure to be tested and then monitoring the low-level sounds which occur as a consequence of deformation, cracking, or other mechanical disturbances occurring in the specimen being tested. When the specimen is strained beyond its elastic limit, it emits a characteristic noise signal called an "acoustic emission". Detection of acoustic emission signals allows a prediction as to when the material is about to fail, and gives an opportunity to prevent the failure. It is the rapid release of kinetic energy from the deformation mechanism that propagates elastic waves from the source, and which are detected as small displacements on the surface of the material. Generally, these deformation processes include plastic flow, fracture and phase transformations. Stress corrosion cracking has also been studied by this technique, but as in other crack-formation phenomena, it is the relaxation of stress which generates the elastic waves. The present invention is a novel extention of prior concepts into the area of non-mechanically generated sounds and is directed to the non-destructive inspection for chemically induced corrosion.
Heretofore, inspection for corrosion damage has relied on such non-destructive techniques as those employing eddy currents, ultrasonics, and penetrants. Also, certain chemical techniques have been employed for corrosion detection. However, such techniques have been limited particularly with respect to their sensitivity and ability to measure small amounts of corrosion or to predict failure from the measurement of corrosion rates.